An impromptu transcontinental race has begun, with nothing less at stake than the future of the American political and governmental system.

It began a few months ago when the Supreme Court, in a 5-4 decision, lifted many of the historically entrenched restrictions on corporate and labor union financing of election ads. Now, the Democratic Congress is attempting to mitigate the ruling’s effects by legislating tough new disclosure requirements on that money — and it is running into predictable interest group opposition.

Meanwhile, California voters last week approved a ballot measure that next year would end the system of party primaries for all offices and substitute a “top two” nominating system in which all candidates would appear on the same ballot. The two most favored, regardless of party, would go on to November.

The new system, tried only in Washington state before now, faces possible court challenges. Opponents — including the leaders of the state Republican and Democratic parties — claim it could fatally weaken their role in election campaigns.

In effect, what we are about to witness is a monumentally consequential new round in the old struggle to balance the cohesive and fractionating forces in this diverse, continental republic.

Early on, the Founding Fathers worried about the splintering capacity of “factions,” by which they meant private interests of all kinds. In the industrial age, that danger focused on corporations and, later, on labor unions. The result was a series of enactments, culminating most recently in the McCain-Feingold campaign finance bill, restricting the flow of private funds to federal campaigns.

Then, this year, the Supreme Court narrowly found that some of those restraints violated the free speech rights of corporations and struck them down. It is not clear how eager those groups are to buy more leverage in elections, but Democrats are not eager to find out — and are trying to move legislation that would, in effect, raise the price for such groups by forcing them to publicize any role they play.

While the battle to expand or contain interest-group influence unfolds in Washington, California — that national trendsetter — has become Armageddon for the political parties.

Reformers, led by Gov. Arnold Schwarzenegger, frustrated by the polarization that hobbles the Legislature as much as it does Congress, turned to the “top two” system in hopes it will lead to more victories for moderates, whatever their party label.

The theory is that if all voters get to screen all candidates at primary time, rather than partisans selecting from fellow partisans, then those on the ideological extremes may be weeded out.

But the theory is largely untested, and the costs could be high. Minor parties and independents and mavericks would likely have a harder time finding a path to the November ballot.

Moreover, leaders of the two major parties fear that without an enforceable party role in the nomination of candidates, the struggle to counter the divisive forces represented by wealthy interest groups and self-financed, ambitious or famous individuals will not just infect politics but dominate it. They argue that, especially in light of the Supreme Court decision, elections could become auctions.

I think our history suggests that the cohesive power of parties is the only real offset to the narrower agendas of interest groups — as we witnessed in the recent health care fight. It takes a strong party to overcome the lobbies.

The latest chapter in this historic struggle has just begun, and it is possible that those who are trying to limit the influence of lobbies and strengthen the political parties will find ways to recoup. But for those of us who think that “faction” is the danger and that political parties are at least part of the solution, this is a scary moment.

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